Plate tectonics

Question 1

Trenches

Answer 1

Island arcs (e.g. Japan) or continental mountains (e.g. Cascades) on the concave side. Active volcanoes have andesitic composition.

Origins of the largest earthquakes on Earth on thrust faults.

Big gravity anomalies as not in isostatic equilibrium. Shape maintained by flexure.

Question 2

Rate of subduction

Answer 2

mean: 70mm/yr

up to 100mm/yr (e.g. Japan)

Question 3

Attenuation and seismic velocity in subducting slabs

Answer 3

Low attenuation (damping)
High seismic velocity (colder than surrounding mantle). In Japan, subducting slab has P and S-wave velocities that are 6% higher than surrounding mantle

Question 4

Andesitic volcanoes

Answer 4

Above where earthquake zone reaches depth of 100km

Question 5

Olivine –> wadsleyite

Answer 5

410km depth

Question 6

ringwoodite –> perovskite

Answer 6

650km

Question 7

wadsleyite –> ringwoodite

Answer 7

520km

Question 8

Earthquakes along narrow zones

Answer 8

narrow, localised bands of deformation

Plates don’t deform at their centres, so are rigid

Question 9

Sound released during an Earthquake

Answer 9

Varies with azimuth (direction from focus to seismic station measure clockwise from North).
Orientation and type of fault can be identified.

Question 10

Transform faults

Answer 10

‘horizontal’ motion on a sphere is equivalent to angular rotation about an axis through the sphere. Euler poles are where the axes exit the sphere.
Horizontal motion follows small circles about the axis.
Euler pole describing motion of transform faults is perpendicular to their strike.
Since the same Euler poles describes motion of all transform faults, plates are rigid.

Question 11

Rigid plates

Answer 11

Deformation only at plate boundaries.

Same Euler pole for all transform faults:
Horizontal direction of slip should all be parallel.

Geodesy:
Surveying the planet. Mostly done with GPS triangulation nowadays. Capable of mm precision over hundreds of km.
Shows accurate measurement of relative plate motion
Show that plates are rigid at <1mm/yr level. Interiors don’t deform.

Question 12

Mercator projection

Answer 12

All lines of lattitude are parallel

Question 13

Back-arc basins

Answer 13

Slabs fall vertically and cause extension of the continental crust. Will first get sedimentary basins and then the crust will rift permanently.

Question 14

What determines speed of motion of a subducting plate

Answer 14

Size of the plate is insignificant. Shear stresses at base of plate are insignificant. Likely the LVZ at the base of plates is also a low viscosity zone.

Length of the subduction zone is significant. Slab pull is a significant force driving plate motion.